[CVE-2026-74692][MODERATE 7.0] net/smc: fix TOCTOU race between smc_listen_out() and listener close [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-74692
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/smc: fix TOCTOU race between smc_listen_out() and listener close [ Upstream
Commit: 53c7938d8bcfde3296ec1a347ba2a9393c1fdcfa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=53c7938d8bcfde3296ec1a347ba2a9393c1fdcfa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74692
Analysis date: Sat, 22 Aug 2026 15:50:08 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A TOCTOU race in the SMC listener close path can allow a remote peer to enqueue a child socket on a closed listener, leaking socket references and potentially exhausting kernel memory during repeated connection attempts.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-74692 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74692

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE 7.0
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-74692	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-367;CWE-362;*CWE-401;**CWE-400;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'A TOCTOU race in net smc listener handling allows smc_listen_out() to test the listener state before taking the listener lock. If the listener is closed in the window between the unlocked state check and smc_accept_enqueue(), a child socket can be enqueued on a dead listener and the sock_hold reference taken by smc_accept_enqueue() is not released. A remote peer can contribute to triggering this by opening TCP connections to an SMC listener while the server closes or restarts the listener, causing leaked smc_sock and clcsock objects and eventual kernel memory exhaustion. For the CVSS the PR:N is used because the attacker only needs network reachability to the exposed SMC service and does not need a local account on the victim. The issue is network reachable but timing dependent because the close path must race with handshake work already running on smc_hs_wq. Impact is denial of service via resource exhaustion. No direct confidentiality or integrity primitive is evident from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	RACE LEAK IMPROVEONLY TEST  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative score signals:

* Remote reachable / network-triggerable: +2
  A remote peer can open TCP connections to an exposed SMC listener and participate in the CLC handshake path.

* Real lifetime corruption: +1
  The bug causes a reference lifetime imbalance. `smc_accept_enqueue()` takes a `sock_hold()` on the child socket, but the child is enqueued on a closed listener and the reference is not released.

* Reliable kernel crash / strong DoS: +1
  Repeated triggering can leak `smc_sock`, `clcsock`, and references until kernel memory is exhausted.

* Hard or unreliable race / special timing required: -1
  The issue requires a TOCTOU window between `smc_listen_out()` and listener close.

* Availability impact realistic: +1
  Sustained remote connection attempts during listener close or restart can cause system-wide memory pressure or eventual DoS.

Paranoid adjustment:

* Paranoid score does not apply the full race penalty because the race can be naturally amplified by repeated remote connection attempts during service restart or listener teardown.

## 3. Reachability analysis

The attacker needs network reachability to an SMC service. No local account on the target is required, so the practical privilege requirement is remote unauthenticated access to the exposed listener. The bug is not in the generic TCP receive path, but it is in a network-facing protocol path once SMC is enabled and listening.

Namespaces and containers do not materially reduce the remote attack requirement, but local containerized workloads could also trigger similar behavior if they can expose or control SMC listeners. SMC is not universally deployed by default, so exposure depends on whether the system runs SMC-enabled services. Realistic exploitation also depends on racing listener close or restart with incoming connections.

## 4. Severity interpretation

This behaves like an actionable Moderate network DoS issue, not an Important memory corruption issue. The patch shows a TOCTOU race and a reference/resource leak, but not UAF reuse, attacker-controlled reclaim, type confusion, arbitrary write, or information disclosure.

The realistic impact is kernel memory exhaustion and denial of service. The theoretical risk does not justify privilege escalation scoring because the leaked objects remain referenced rather than freed and reused.

## 5. One-sentence report phrase

A TOCTOU race in the SMC listener close path can allow a remote peer to enqueue a child socket on a closed listener, leaking socket references and potentially exhausting kernel memory during repeated connection attempts.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network reachable and can lead to kernel memory exhaustion, although it is race-dependent and currently appears to be DoS-only rather than a privilege escalation primitive.

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: net/smc: fix TOCTOU race between smc_listen_out() and listener close [ Upstream
Commit: 53c7938d8bcfde3296ec1a347ba2a9393c1fdcfa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=53c7938d8bcfde3296ec1a347ba2a9393c1fdcfa

Changed files:
  net/smc/af_smc.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=53c7938d8bcfde3296ec1a347ba2a9393c1fdcfa

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-74692 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74692

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 4
  Paranoid ActionableScore: 5

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).